1. Field of the Invention
The present invention relates to a data transmission system for transmitting QAM data over an analog line such as a telephone line, and more particularly, to an apparatus method for canceling periodic carrier phase jitter over transmission lines, in a data modem receiver.
2. Description of the Related Art
Digital data transmission using data modem receivers suffers transmission error caused by carrier phase jitter that is produced over the transmission line, resulting in a reduction in the data transmission efficiency. Various efforts have been made to reduce the phase jitter within data modem receivers.
Generally, apparatus for canceling carrier phase jitter comprises a carrier reproducing circuit for absorbing the carrier phase jitter, the carrier reproducing circuit comprising a phase-locked loop in the data modem receiver. However, as the response speed of the carrier reproducing circuit is increased for greater carrier phase jitter absorption, the carrier reproducing circuit becomes more sensitive to thermal noise from a transmission line. As the known result, the apparatus for canceling carrier phase jitter are degraded in-their overall characteristics.
Another known jitter canceler is disclosed in U.S. Pat. No. 4,639,939 assigned to NEC Corporation. The disclosed jitter canceler is more capable, in principle, of separating carrier phase jitter and thermal noise. However, since the jitter canceler is basically composed of an FIR filter, the scale and complexity of the circuit arrangement tends to be increased in order to provide sufficient jitter canceling characteristics.